Proc. Natl. Acad. Sci. USA Vol. 84, pp. 7218-7220, October 1987 Genetics High rate of recombination and double crossovers in the mouse pseudoautosomal region during male meiosis (sex chromosomes/steroid sulfatase/Mov-15/transgenic mice/crossover interference) PHILIPPE SORIANO*t¶, ELISABETH A. KEITGESt, DANIEL F. SCHORDERETt, KLAUS HARBERS§, STANLEY M. GARTLERt, AND RUDOLF JAENISCH*t *Whitehead Institute for Biomedical Research, Nine Cambridge Center, Cambridge, MA 02142; tDepartment of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139; tDepartments of Medicine and Genetics, and Center for Inherited Diseases, SK-50, University of Washington, Seattle, WA 98195; and §Heinrich-Pette-Institut fur Experimentelle Virologie und Immunologie an der Universitat Hamburg, Martinistrasse 52, 2000 Hamburg 20, Federal Republic of Germany Communicated by Gerald R. Fink, July 13, 1987 ABSTRACT The recombination rate in meiosis between To further study recombination in the mouse pseudo- the mouse X and Y chromosomes was analyzed. Mice hetero- autosomal region, we have examined the linkage between Sts zygous at two pseudoautosomal alleles, the steroid sulfatase and another pseudoautosomal marker, the Mov-15 provirus gene and the Mov-15 provirus marker, were crossed. The carried in a transgenic mouse strain (9). We have previously provirus in the Mov-15 transgenic mouse strain had been shown that the Mov-15 provirus recombines with Tdy in previously shown to be carried in the pseudoautosomal region 10-20% of male meioses. We describe here linkage analysis of the sex chromosomes. Recombination frequencies were between Mov-15, Sts, and Tdy. Our results show that the shown to be 7-fold higher in this region in male meiosis than in frequency ofrecombination between Sts and Mov-15 is 7-fold female meiosis. Three-point crosses indicated the occurrence in higher during male meiosis than during female meiosis, that male meiosis of double recombination events in the pseudo- there is a gradient of sex linkage in the mouse pseudo- autosomal region, with little or no crossover interference, autosomal region, and that, in contrast to the human sex which is in marked contrast to observations made on the similar chromosomes, double recombination events take place with region of the human sex chromosomes. This result is contrary little or no crossover interference. to a previous model, which predicted a single crossover event in male meiotic pairing of mammalian sex chromosomes. MATERIALS AND METHODS Pairing of mammalian X and Y chromosomes occurs during Mice and Assays. C3H/An steroid sulfatase-negative ani- meiosis in a short region located near the telomeres. Alleles mals and Mov-15 animals (BALB/c background) have been in this region do not show strict sex-linked inheritance described (4, 9). Steroid sulfatase activity was determined on because they can undergo reciprocal exchange between the ear punches as described (4) using [3H]estrone sulfate or sex chromosomes and have therefore been termed pseudo- [3H]dihydroepiandrosterone sulfate as substrates and stan- autosomal (1, 2). A gradient of sex linkage and a higher dardizing for the amount of protein. Because the presence of frequency of recombination during male meiosis than in the provirus has been shown to lead in all cases to the female meiosis have been established in the human pseudo- establishment of viremia (9), offspring from viremic fathers autosomal region (3). To explain the lack of sex linkage of could be screened by radioimmunoassay for the viral antigen distal markers, a crossover has been p30. Because maternal transmission of the provirus by milk single obligatory pos- can lead to the establishment of viremia, offspring resulting tulated to occur in this region (2, 3). Similar studies in the from viremic mothers were genotyped by blot analysis of tail mouse have not yet been performed because few DNA using a viral-specific probe to reveal hybridization pseudoautosomal markers are available. In a previous study, bands diagnostic for the Mov-15 provirus (Fig. 2; ref. 9). All information on recombination in the mouse pseudoautosomal offspring found to be recombinants were confirmed by two region was obtained by investigating the linkage between the independent assays for steroid sulfatase activity and for the mouse steroid sulfatase gene (Sts), a pseudoautosomal mark- presence of the provirus. er that does not show sex linkage and therefore should be located in the distal pairing region (4), and the sex-reversed mutation (Sxr; see refs. 5 and 6; Fig. 1). The two markers RESULTS AND DISCUSSION were shown to be linked, and one double recombinant The genetic crosses are outlined in Fig. 3. Females homozy- between these two markers and sexual phenotype was gous for the Mov-15 proviral insertion were crossed with identified among 59 progeny tested (7). This suggested that steroid sulfatase and provirus negative (Sts-; Mov-) animals. the single obligatory crossover model may not be applicable Resulting F1 offspring were further crossed with (Sts-; to the mouse sex chromosomes. However, because Sxr is a Mov-) animals. Male and female offspring of the F2 gener- translocation to the telomere ofthe part ofthe Y chromosome ation were typed for steroid sulfatase activity and for the bearing Tdy, the testis-determining gene (for review, see ref. presence of the provirus as described in Materials and 8), these crosses could not examine recombination in the Methods. Analysis of the results obtained by crossing female pseudoautosomal region of the normal Y chromosome. F, animals provided information on recombination during Moreover, in these crosses, recombination between Sts and female meiosis between the two pseudoautosomal markers Sxr in female meiosis could not be analyzed because XX Mov-15 and Sts. Analysis of the results obtained by crossing animals carrying the Sxr mutation were sterile males. the male F1 progeny provided information on recombination between three markers, Sts, Mov-15, and Tdy, which is The publication costs of this article were defrayed in part by page charge payment. This article must therefore be hereby marked "advertisement" Present address: Institute for Molecular Genetics, Baylor College of in accordance with 18 U.S.C. §1734 solely to indicate this fact. Medicine, Houston, TX 77030. 7218 Downloaded by guest on September 27, 2021 Genetics: Soriano et al. Proc. Natl. Acad. Sci. USA 84 (1987) 7219 1 2 3 4 5 6 7 8 Region of strict 1 y Origin of Sxr sex 11inkage Region of Pairing and | | Mov 15 Recoinhination with I I 32M- a 4W tile X chromosome 1 Sts l I FIG. 1. Diagram of the mouse Y chromosome. The positions of the three markers used in this study-Tdy, Mov-15, and Sts-are indicated. The drawing is not to scale. carried on the Y chromosome, constituting a three-point cross (Fig. 3). The results of these crosses are presented in Table 1. In male meiosis, recombination between Sts and Tdy was 42%, confirming previous results that showed the absence of significant sex linkage for this marker (4). Because the FIG. 2. DNA blot analysis of Mov-JS offspring. Genomic DNA Mov-15 provirus was inherited in the males of the F1 was isolated from the terminal third of tails, digested with Kpn I, and generation from their mothers, the provirus must be located blot hybridized with a viral-specific probe as described (9). Animals on the X chromosome. Analysis of the progeny of F1 males 2-4 were positive for the Mov-15 provirus, as shown by the presence showed that 20% of the offspring were either males contain- of viral-specific bands representing the 3' and 5' ends of the viral ing the proviral genome, or females without the proviral genome. 82m is a 82-microglobulin probe (a gift of Mike Gilman), which serves as an internal control for the amount of DNA loaded in genome, indicating recombination between the Mov-15 mark- each lane. er and Tdy. This recombination rate was similar to previous results (9). The recombination frequency between the two pseudoautosomal loci, Sts and Mov-15, was found to be 27%. linkage observed in the pseudoautosomal region must corre- In female meiosis, recombination between these two markers late with the linear order of loci within this region. This is was found to be a factor of 7 lower (4%). Three double consistent with the following linear order: Sts, Mov-15, Tdy, recombinants were also detected among the 127 offspring and centromere. If genetic distances are proportional to tested. Such offspring would be expected to be Sts-negative physical distances, our results would assign the Mov-15 locus males carrying the Mov-15 provirus or Sts-positive females to the more proximal part and Sts to a distal part of the not carrying the provirus (see Fig. 3). One such male and two pseudoautosomal region. such females were observed. To account for the absence of sex linkage of distal A significantly higher frequency of recombination during pseudoautosomal markers, it has been postulated that a male meiosis than during female meiosis has also been single obligatory crossover must take place in this region documented in the human pseudoautosomal region, which is during male meiosis (2). Support for this model was provided located on the distal short arm of the sex chromosomes (3). by a previous study of the human pseudoautosomal region, Koller and Darlington had already postulated many years ago where no double crossovers were observed among three (10), on the basis of cytological observations, that chiasma different pseudoautosomal loci (3). This result, however, formation and subsequent recombination were necessary for may have been at the limit of statistical significance, since normal disjunction ofchromosome pairs, including the X and only 2.4 such events could have been expected on the basis the Y chromosome. The high frequency of recombination in of the linkage between the pseudoautosomal markers in- the pseudoautosomal region that we have observed during volved.